This invention relates to positron annihilation techniques for studying defects in metals and especially to positron probes for use in the application of these techniques to nondestructive testing.
There is a continuing need for new techniques for the nondestructive testing and evaluation of mechanical components for both military and civilian use. Materials that must withstand severe mechanical and/or thermal stress cycling are subject to fatigue damage. In many cases, such as in aircraft parts or gun barrels, fatique failure can be catastrophic.
Conventional nondestructive testing methods, such as X-ray, electrical resistance, magnetic perturbation, ultrasonic, and holographic measurements, are not sensitive enough to detect fatigue damage until after small cracks have opened, which can be long after reasonable safety margins have been exceeded. X-ray techniques require precision geometric relationships to be maintained between the material being studied and the X-ray source and/or detector. Ultrasonic and acoustic holographic techniques may require the item being studied to be immersed in a suitable energy coupling medium such as water.
The need for reliable fatigue-life predictions and determinations for large-bore gun barrels will become increasingly important as chemical milling techniques allow the use of very wear-resistant brittle alloys for gun barrel fabrication. Fatigue life will become more important than wear life. If a technique were available for the periodic in situ monitoring of the extent of fatigue damage in a gun barrel, the very expensive replacement of these items could be scheduled in accordance with actual damage measurements, and the full safe fatigue life of a barrel could be utilized.
The use of positron annihilation techniques for research studies of defects in metals is well established. (For excellent review articles, see "Positron Annihilation Techniques, PAT, in Polymer Science and Engineering," Journal of Macromolecular Science-Reviews of Macromolecular Chemistry, Volume C9(2), pages 305-337, 1973, by Hameleck, Eldrup, Mogensen and Jansen, and, "Studies of Lattice Defects by Means Of Positron Annihilation", in Crystal Lattice Defects, Volume 4, pages 139-163, 1973, by Doyama and Hasiguti). Successful fatigue damage studies have been performed under laboratory conditions on specially prepared samples. However, the development of positron techniques for in situ fatigue damage measurements on highly stressed structures will depend in part on the development of suitable positron source probes. These probes would permit fatigue detection measurements to be performed on mechanical parts such as turbine blades, aircraft landing gears, helicopter rotor hubs, gun barrels and mounts.
An object of this invention is to fill the need for positron probes to be used in positron annihilation techniques for nondestructive testing applications.